There are things man was not meant to know.
Unless you know these things, you'll never be able to confront the eldritch horrors of the unknown.
The more you know of them, the more distant you become from the rest of humanity.
The more distant you become from the rest of humanity, the more chance you'll get locked up as a lunatic, or simply break down in a heap of anguish.
There it is, the basic premise behind the works of HP Lovecraft, and the core ideas behind the mechanisms of Sanity in Chaosium's Call of Cthulhu.
Onto this raw driving force was tacked a percentile skill system and a bunch of subsystems that I've rarely seen in use. But the core is what drives a game of Call of Cthulhu, whether it's a single session witch hunt, or a growing symphony of horror.
The Sanity system integrates into the percentile skill system, I wouldn't say that it dovetails into it (it seems a little clunky after 30 years), but it still works.
Like most games of the era, it used 3d6 to generate most statistics, then derived some values using other means to get the figures actually used in play...
Your Sanity starts at a certain level (five times you POW score), but if your combined Sanity and Cthulhu Mythos score ever exceeds 100, you're sanity drops until the total is 100 points.
Sanity is a very valuable trait as it lets you fend off horrors by not believing in them, Cthulhu Mythos is a very valuable skill because it teaches you how to fight off monsters that you can't disbelieve.
The whole game is about confronting horrors, so the core mechanism reflects this.
It gives a narrative choice and a mechanical choice, both of which mirror one another. Do you choose to have faith in humanity and the light (and thus pursue tactics relevant to this agenda), or do you choose the occult and darkness (and thus risk taint or ostracism from the mundane world).
From a perspective of Vector Theory, the wavelength and spectrum of the character retains a maximum intensity, but the colouring changes...as the Cthulhu mythos part of the spectrum increases, the sanity part of the spectrum decreases. The Cthulhu Mythos part of the spectrum is alluring because it offers power and new supernatural choices, but the sanity part of the spectrum is almost impossible to regain once it's lost. And it's Sanity that works as the best defence against the creatures of the night. Too little of it and you'll easily succumb to the horrors of the game.
The whole game has a general bias in this way.
Every choice you make has an easy option that brings you closer to temptation, and a hard option that might actually make a difference in the end.
Every filter passed, has a chance of reducing sanity further. But only a few rare filters offer the chance to regain it.
The polarisation of the game usually lends itself to dark and moody drama. If you're playing Call of Cthulhu, you know what to expect. If it doesn't have the darkness, the horror and the unknown, it just ain't CoC.
The game also has a gravity well. A fall to the unknown.
Narratons are drawn to a gravity well, like photons. It is a natural conclusion. In this game the characters often have to work hard to avoid succumbing to the unknown. Unless they make conscious decisions to avoid fate, they WILL be driven insane or they WILL die at the hands of eldritch beasts and dark gods. If they leave it too late to make a stand, they can only choose a method of demise, they can't avoid it completely.
But other tactics also become apparent when looking at the game in this way. They might be able to charge headlong into the finale, hoping to cause enough of a bang to pass through the other side, they might be able to deflect their trajectory enough to wrap around the gravity well sucking them in, catching a glimpse of the horror before hurtling back into safer territory.
There are always options, and a good GM will take these options for what they are...or will have considered contingency plans should they arise.